1. Field of the Invention
The present invention relates to a holding control valve for use in, for example, a hydraulic control circuit which activates an activated body (cylinder device) of a hydraulic operation apparatus, such as a hydraulic shovel.
2. Description of the Related Art
A typical related art is disclosed in, for example, Patent Document 1, Japanese Patent No. 3919399. Patent Document 1 discloses a hydraulic control circuit for controlling the hydraulic operation apparatus, such as the hydraulic shovel, including the holding control valve. This hydraulic control circuit includes: a pump; a control valve communicated with the pump; a pilot check valve connected to the control valve via a load supporting pipe; and a cylinder device having a pressure chamber connected to the pilot check valve.
The pilot check valve is provided with a back pressure chamber which is communicated with the pressure chamber of the cylinder device. The control valve is configured to block the cylinder device from the pump when it is at a neutral position, introduce discharged oil of the pump to the pressure chamber of the cylinder device to move up a load when it switches to an up position, and discharge the hydraulic oil of the pressure chamber of the cylinder device to move down the load when it switches to a down position. Further, a pilot pressure controller (operating lever) is provided which controls a pilot pressure by which the control valve switches to the up position or the down position. When the pressure of the back pressure chamber of the pilot check valve is a load pressure of the pressure chamber of the cylinder device, the flow of the hydraulic oil from the pressure chamber side of the cylinder device is blocked by the pilot check valve. When the hydraulic oil of the back pressure chamber of the pilot check valve is discharged, the pilot check valve opens to allow the flow of the hydraulic oil from the pressure chamber side of the cylinder device.
Then, the hydraulic control circuit further includes: a connecting passage which connects between the pressure chamber of the cylinder device and the pilot check valve; a first switching device which blocks the connecting passage when it is a normal state and opens the connecting passage and causes the pressure chamber of the cylinder device and the load supporting pipe to be communicated with each other via a throttle when it is a switched state; and a second switching device which maintains the pressure of the back pressure chamber of the pilot check valve at the load pressure of the pressure chamber of the cylinder device when it is a normal state and discharges the hydraulic oil of the back pressure chamber when it is a switched state, and is configured such that: the first and second switching devices switch by the pilot pressure by which the control valve switches to the down position; only the first switching device switches when the pilot pressure is a predetermined pressure or lower; and both the first switching device and the second switching device switch when the pilot pressure exceeds the predetermined pressure.
Each of the first and second switching devices is constituted by a switching valve configured to causes a spool to operate against a spring force (pressure) by introducing the pilot pressure from the control valve. Moreover, a relief valve is disposed on a branched passage which branches from the connecting passage connecting between the pressure chamber of the cylinder device and the pilot check valve. A relief oil exit side of the relief valve is connected to a drain tank via a throttle member, and the relief valve branches on an upstream side of the throttle member to be communicated with a pilot pressure introducing portion side spool hole of the switching valve.
With this configuration, in a case where an external force is applied to the cylinder device when the load of the cylinder device is maintained at a constant pressure, i.e., when the control valve is maintained at a normal state, the load pressure of the pressure chamber of the cylinder device increases, and this causes the relief valve to operate, so that the relief oil flows out from the relief valve. Pressure rises on the upstream side of the throttle member by the existence of the throttle member on a flow-out side (exit side) of the relief valve. The pressure rising on the upstream side of the throttle member is introduced to the spool hole. An introducing port through which the pressure is introduced from the relief valve to the spool hole is located at an adjacent portion which is adjacent to the spool and the piston located close to the spool. The pressure of the relief oil introduced from the relief valve to the spool hole acts on the spool and the piston at this adjacent portion. Therefore, the spool and the piston move so as to separate from each other, and the second switching device of the switching valve switches, so that the oil of the pressure chamber of the cylinder device is Patent Document 1: Japanese Patent No. 3919399.
FIG. 7 is a diagram conceptually showing the configuration of a portion including the pilot pressure introducing portion and relief oil introducing portion in the switching valve incorporated in the hydraulic control circuit. In FIG. 7, a spool C is fittingly inserted in and supported by a spool hole B of a switching valve A so as to be slidable along an axis line of the spool hole B. A plug D threadedly engages with one end of the spool hole B, so that the end of the spool hole B is sealed. A piston E having a larger diameter than the spool C is disposed between the plug D and the spool C. An airspace between the piston E and the plug D functions as a pressure receiving portion which receives a pilot pressure P introduced from the control valve (not shown). Moreover, relief oil G1 from a relief valve G connected to a cylinder device F is introduced to an airspace between the piston E and the spool C, and this airspace portion functions as a pressure receiving portion which receives pressure at the time of a relief operation. The piston E is provided with a communication passage E1 extending from the pressure receiving portion of the pilot pressure P to the pressure receiving portion used at the time of the relief operation, and the communication passage E1 includes an orifice E2 at a portion thereof.
The configuration of the switching valve and the configuration of the hydraulic control circuit other than the portion including the introducing portions shown in FIG. 7 are substantially the same as those shown in FIG. 2 of Patent Document 1 and those shown in FIG. 1, so that explanations thereof are omitted.
In the switching valve configured as above, the piston E is pressed upward by the introduced pilot pressure P, and the spool C is then pressed by the large-diameter piston E, so that the first switching device switches. If the pilot pressure P further increases to exceed a predetermined pressure, the second switching device also switches together with the first switching device. Moreover, when the relief oil G1 is introduced, the oil pressure of the relief oil G1 is applied to the spool C, so that the second switching device switches. Then, when the cylinder device F becomes a high load (high pressure) state, pre-leakage (phenomenon in which the flow rate of oil flowing therethrough increases at a pressure equal to or lower than a set pressure) occurs in the relief valve G, and pressure loss by the throttle member occurs. Since the oil pressure against the pilot pressure is generated by the occurrence of the pressure loss, the switching valve does not follow the pilot pressure, and opening start points of the first and second switching devices are inappropriate.
FIG. 8 is a graph showing actual measurement results of a relation among the pilot pressure P, a cylinder port pressure, and a flow rate of oil flowing through the switching valve and the pilot check valve when the pilot pressure P is applied in a simulation in which the pressure of 30 MPa is generated on a bottom side of the cylinder device. In FIG. 8, each solid line denotes changes in flow rate of oil flowing through the switching valve and the pilot check valve, and each broken line denotes changes in cylinder port pressure. As can be understood by FIG. 8, in a case where the flow rate increases, it less changes even after the pilot pressure exceeds 1 MPa that is a set value of the opening point, and it starts increasing from about 1.6 MPa, and in a case where the flow rate decreases, it does not become 0 L/min even after the pilot pressure falls below 1 MPa. This means that a following capability of the flow rate with respect to the pilot pressure P is low and the opening start point is inappropriate.